1. Field of the Invention
The present invention relates to devices for monitoring vital signs. More specifically, the present invention relates to devices for monitoring blood pressure.
2. Description of the Related Art
Blood within a patient's body is characterized by a baseline pressure value, called the diastolic pressure. Diastolic pressure indicates a pressure in an artery when the blood it contains is static. A heartbeat forces a time-dependent volume of blood through the artery, causing the baseline pressure to increase in a pulse-like manner to a value called the systolic pressure. The systolic pressure indicates a maximum pressure in a portion of the artery that contains a flowing volume of blood.
Pressure in the artery periodically increases from the diastolic pressure to the systolic pressure in a pulsatile manner, with each pulse corresponding to a single heartbeat. Blood pressure then returns to the diastolic pressure when the flowing pulse of blood passes through the artery.
Both invasive and non-invasive devices can measure a patient's systolic and diastolic blood pressure. A non-invasive medical device called a sphygmomanometer measures a patient's blood pressure using an inflatable cuff and a sensor (e.g., a stethoscope) that detects blood flow by listening for sounds called the Korotkoff sounds. During a measurement, a medical professional typically places the cuff around the patient's arm and inflates it to a pressure that exceeds the systolic blood pressure. The medical professional then incrementally reduces pressure in the cuff while listening for flowing blood with the stethoscope. The pressure value at which blood first begins to flow past the deflating cuff, indicated by a Korotkoff sound, is the systolic pressure. The stethoscope monitors this pressure by detecting strong, periodic acoustic ‘beats’ or ‘taps’ indicating that the blood is flowing past the cuff (i.e., the systolic pressure barely exceeds the cuff pressure). The minimum pressure in the cuff that restricts blood flow, as detected by the stethoscope, is the diastolic pressure. The stethoscope monitors this pressure by detecting another Korotkoff sound, in this case a ‘leveling off’ or disappearance in the acoustic magnitude of the periodic beats, indicating that the cuff no longer restricts blood flow (i.e., the diastolic pressure barely exceeds the cuff pressure).
Low-cost, automated devices measure blood pressure using an inflatable cuff and an automated acoustic or pressure sensor that measures blood flow. These devices typically feature cuffs fitted to measure blood pressure in a patient's wrist, arm or finger. During a measurement, the cuff automatically inflates and then incrementally deflates while the automated sensor monitors blood flow. A microcontroller in the automated device then calculates blood pressure. Cuff-based blood-pressure measurements such as these typically only determine the systolic and diastolic blood pressures; they do not measure dynamic, time-dependent blood pressure.
Time-dependent blood pressure can be measured with a device called a tonometer. The tonometer typically includes a sensitive transducer that contacts a patient's skin and compresses an underlying artery against a portion of bone. The transducer then measures blood pressure in the form of a time-dependent waveform. The waveform features a baseline that indicates the diastolic pressure, and time-dependent pulses, each corresponding to individual heartbeats. The maximum and minimum values of each pulse are, respectively, the systolic and diastolic pressures. The rising and falling edges of each pulse correspond to pressure values that lie between the systolic and diastolic pressures.
Data indicating blood pressure are most accurately measured during a patient's appointment with a medical professional, such as a doctor or a nurse. Once measured, the medical professional manually records these data in either a written or electronic file. Appointments typically take place a few times each year. Unfortunately, in some cases, patients experience ‘white coat syndrome’ where anxiety during the appointment affects the blood pressure that is measured. White coat syndrome typically elevates a patient's heart rate and blood pressure; this, in turn, can lead to an inaccurate diagnoses.
Blood pressure monitors can also be used in an ambulatory setting to counteract white coat syndrome. In this case, the monitors are worn by patients over a short (e.g., 24-hour) period, and are programmed to make periodic measurements (e.g., once every hour). Some ambulatory blood pressure monitors include systems for transmitting data from a remote site, such as the patient's home, to a central database. These systems can include a conventional computer modem that transmits data through a telephone line to the database. Or alternatively they can include a wireless transmitter, such as a cellular telephone, which wirelessly transmits the data through a wireless network.
Monitoring a patient's blood pressure, particularly in an ambulatory setting, can be difficult since a measurement may be taken when the patient's limb is in motion, thereby skewing the reading. It would therefore be desirable to have a device that indicates when the patient's limb is at rest and at an optimal time to take a blood pressure measurement.